The CausesThe main causes for deck collapses are either post and railing failures, or the connection of the deck ledger board to the house’s rim joist. The ledger board is usually a 2 by 8 or 2 by 10 pressure treated board that is bolted or screwed to the house and holds the joists of the deck. It is estimated that 80 percent of the deck collapses occur at the ledger to rim joist connection.
Deck ledgers should be screwed or preferably bolted to the building rim joists not merely nailed. Properly installed, bolts and lag screws will withstand the expansion and contraction of the wood. They can loosen over time, but will not pull out as nails would. You will notice a gap between the deck and the house is the bolted or screwed connection is loosening. If the ledger board has been nailed to the house, any lateral movement may cause the deck shift to the point of collapse.
Lag bolts and screws must be spaced properly to maintain the deck’s connection to the house. If bolts are spaced too far apart, they can bend if the deck becomes overloaded or is loose. Thirteen people died in a deck collapse in Chicago due to improperly spaced lag bolts.
If the connection between the ledger board and rim joist is not flashed properly, or if flashing was not installed at all, water can get behind the ledger board and cause the rim joist on the house to rot. This is because rim joist are not pressure-treated and will be susceptible to rot if they remain wet over long periods of time Flashing can be installed in the form of either a metal coil cut to the length of the ledger board and bent to fit under the siding and cover the top edge of the board, or in pre-formed flashing that comes in various lengths. Aluminum flashing should not contact pressure treated wood due to a corrosive chemical reaction of these two materials causing the aluminum to dissolve completely over time. New pressure treated wood uses alkaline copper quaternary (ACQ) chemicals for preservation, which is even more corrosive then previous treatments that were applied to wood. You should make sure that the correct anchors were used for attaching the posts to the pier and that cast aluminum porch post supports are not being used. This change in chemical treating also does not allow for making repairs to existing decks. ACQ in new wood will corrode existing connectors within three years. Decks need to be replaced in their entirety once decay has been observed.
Life Safety - Railings
Railings are one of the most obviously safety issues with decks and balconies because it is readily apparent when they are loose. There are many guidelines for railing design and requirements for installation which include height, grasping ability, design loads in both vertical and horizontal direction, space between railings, and even spindle location. Railings also need to be securely fastened to the deck framing with bolts that go completely through the post into a beam or joist. It is unacceptable to notch posts, so make sure that the deck’s support posts are separate from the railing posts. It is important that you inspect the railings at least twice a year and even a sound deck may have problems with the railings that need to be corrected.
The wood used to construct decks should last 20 to 25 years if properly maintained, but decks that are shaded can degrade faster as the wood does not dry out. This process is made worse if the wood has not been stained or painted. In cases such as this the wood loses it structural integrity as well as the ability to hold a fastener.
As wood ages it will expand and contract, causing graying, splintering, and cupping. This is also a sign of the wood losing its structural capabilities and nearing the end of its life cycle. The older the wood is, the more nails and screws will be pulled out from the decking and joists.
Make sure you have a qualified engineering firm that is competent in analysis of this kind, such as Engineering Express, to ensure appropriate protection and/or remediation.
Posts
